Will Steffen

One of the most direct effects of climate change is the increase in very hot days and heatwaves. Many regions of Australia have suffered unusually intense heatwaves this summer.

Melbourne sweltered through two heatwaves, and on January 16 Australia's extreme heat made front-page news around the world when the Australian Open tennis tournament was forced to suspend matches due to the risk of heat stress to players.

Heatwaves have also increased extreme bushfire danger weather, a factor in the outbreak of fires that have claimed homes in Victoria and South Australia and have burnt large areas of Victoria's Grampians. These events follow the record-breaking summer of 2012-13, in which extreme heat affected 70 per cent of the continent and high temperature records were set in every state and territory.

As the Climate Council's report, released today, shows, heatwaves in Australia are becoming hotter and longer, they are occurring more often, and they are starting earlier in the season. The increase in extreme heat across Australia over the past two decades has been extraordinarily large. For example, the hot weather that residents of Adelaide, Canberra and Melbourne are now experiencing was not projected to occur until 2030.

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A comparison of heatwaves during the most recent 30 years (1981-2011 averages) with those for the 1950-1980 period show in more detail how extreme heat is on the rise in our cities.

In Melbourne, heatwaves are occurring 17 days earlier, they have become 1.5 degrees hotter, and the maximum temperature of the hottest day is 2 degrees higher.

The underlying driver for the increase in extreme heat is the rise in greenhouse gases in the atmosphere, primarily from the burning of fossil fuels. These gases trap more heat and stack the odds towards more hot weather in Australia and around the world.

Even the small increase of 0.85 degrees in global average temperature results in a surprisingly large increase in extreme heat. Since 1950 the number of record hot days across Australia has more than doubled. Over the past decade, the frequency of record hot days has been more than three times the frequency of record cold days.

Heatwaves can have a huge human cost. In Australia, heatwaves cause more deaths than any other natural disaster and are known as ''silent killers''. They affect the most vulnerable in our communities - older people, the very young, and those with pre-existing medical conditions.

Extreme heat can also restrict the productivity of workers, particularly those working outdoors or in heat-exposed jobs. As heatwaves become even hotter and occur more often, decreased labour productivity will become even more costly and will likely force changes in the workplace, such as a transition to working night shifts.

Heatwaves place pressure on infrastructure, with electricity transmission and urban transport systems especially vulnerable. The 2009 Melbourne heatwave caused disruption of the urban rail system and led to widespread blackouts, with about 500,000 people losing electricity.

The impacts extend beyond people to natural ecosystems and wildlife. Over the past decade more than 30,000 flying foxes have died during heatwaves along the east coast of Australia, and in 2010 a heatwave in Western Australia led to the deaths of more than 200 of the endangered Carnaby's black cockatoo.

Marine ecosystems are also at risk from heatwaves. Extreme heat in the surface ocean waters causes corals to bleach and, if bleaching occurs too frequently, corals die. The Great Barrier Reef, one of the world's most iconic natural ecosystems and Australia's most well-known international tourist destination, has been bleached repeatedly since the 1970s. Increasing marine heatwaves and rising ocean acidity, coupled with several non-climate related stresses, have driven widespread death of corals across the reef, with a 50 per cent loss of coral cover in just the past 30 years.

The science is clear. Our ability to reduce greenhouse gas emissions will strongly influence the extent to which extreme hot weather will increase through this century. It is crucial that communities, emergency and health services, and businesses and industry prepare for the increasing severity, intensity and duration of heatwaves. Indeed, a number of positive steps have been made in this direction already.

The choices that we make over this decade will determine the severity of extreme heat that our children and grandchildren experience. To stabilise the climate and limit the increase in heatwaves, greenhouse gas emissions need to be cut urgently and deeply. This is the critical decade to get on with the job.

Will Steffen is an adjunct professor at the Fenner school of environment and society at ANU and is lead author of the Climate Council's report, Heatwaves: Hotter, Longer, Earlier and More Often.